The present invention relates to a projection lens used in optical apparatuses such as an image projection apparatus.
A projection lens used in an image projection apparatus such as a projector requires a long back focus to avoid interference of the projection lens with a color separating/combining optical system. The projection lens for the projector therefore often adopts a so-called retrofocus type lens configuration including a stronger negative refractive power at a position closer to a screen than an aperture stop.
However, the retrofocus type lens is characterized in that, mainly from its asymmetry, field curvature is easily caused due to distance variations, in other words image quality is easily changed due to variation of its projection distance. Moreover, recently, because of the demand for shorter projection distances, the field angle of the projection lens is increased and thereby the characteristic described above becomes more notable. In addition to this, there is less tolerance for influences of variation of the field curvature due to the distance variation because of a higher resolution of an image-forming element such as a liquid-crystal panel.
There is a method called floating for reducing the variation of field curvature. With the floating method, plural lens units are simultaneously moved in an optical axis direction in focusing to mutually cancel field curvatures generated by the plural lens units. An example of using the floating method in a projection lens for projectors has been disclosed in Japanese Patent Laid-Open No. 2005-352407.
However, to implement the floating method disclosed in Japanese Patent Laid-Open No. 2005-352407, it is necessary to provide a complex mechanical configuration that includes not only a cam mechanism, but also a helicoid mechanism in the projection lens. For that reason, not only is the projection lens larger in size, but manufacturing variation easily occurs that invites performance variation of the projection lens.